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Odds are pretty good that all of you r images are using the [Basic Graphics Frame] Object Style. Edit the style to include the stroke and it will be appllied to all frames that use the style.
... and that will be a very good lesson for you, as a new InDesign user, about why you should always use styles. Because when you need to make a change when the book is already laid out, it's much easier to change one definition once in one place than it is to change hundreds of different ilttle bits scattered throughout the document.
The "Basic Graphics Frame" and "Basic Text Frame" are really useful, but it'd be better (for your next project) to define your own "MyFrameForPhotos" and "MyFancyLogoFrame" and so on. Because, if Peter is right, every frame that is not a text frame is going to have the Basic Graphics Frame, and if you have any logos or graphics or anything that is not in a text frame, then when you edit the Basic Graphics frame to have a stroke, then every graphics frame is going to have a stroke. The learning curve on this stuff is steep, but once you've climbed up, then those "rather tiresome" jobs become astoundingly easy.
Of course, if you only want to add a stroke to half of your graphics frames, not all of 'em, you could make a simple graphics style that was "JustAStroke" and step through your 115 threads applying that graphic style to the frames that you wanted to have a stroke. Just select all the images of a spread and click once on your JustAStroke graphic style, instead of... um... selecting all of the image frames on a spread, opening the Stroke palette, specifying a value, and hitting Enter. Or whatever